How important is DNS reverse lookup for web sites and mail servers? Is there any real reason to do it?

Its VERY important. Most ISPs wont bother to deliver your email if its doesnt have a PTR record (AOL, etc.)

That would be true if the traffic was originating from the host/dns entry. But if the host just serving it shouldn't be necessary. Well atleast I can't find a reason. Outside of this confusion I have, let me explain my dilemna.

I have control of my domains dns records through my web host. I want to add entries within dns that points back to servers on my local dmz. I know there is no issues with the forward lookup because it will ultimately query my dns to resolve. My problem is where do I set the reverse entry? My web host or my ISP?